2. BACKGROUND
There is continual pressure for integrated circuits to be increasingly faster and increasingly more powerful. Both of these objectives tend to be influenced by the size of the integrated circuits. By fabricating smaller integrated circuits, electrical pathways are shorter and more devices are formed within a given space, which tends to result in a faster, more powerful integrated circuit.
As the size of devices is reduced, it is increasingly difficult to adequately fill and cover the devices with the successive layers of material from which the devices are formed. This is because the small feature size tends to produce surfaces that are extremely uneven. If the extremely uneven surfaces are not modified in some manner, then the overlying layers of material tend to suffer from a variety of problems, such as not conforming to the underlying surface in a uniform manner, forming pinholes and cracks either during deposition or during subsequent processing, and cracking from unbalanced and unalleviated stresses.
These problems, and others, are reduced in large measure by planarizing the substrate in some manner. During a planarization process, material is removed from the surface of the substrate to obtain a substantially flat surface. The planarization process is stopped when the surface of the substrate reaches a target surface. The target surface is generally known as an endpoint. If an endpoint cannot be readily detected, then either too much material or too little material tends to be removed from the surface of the substrate, which may undesirably affect the performance of the integrated circuit.
What is needed, therefore, is a method for planarizing the surface of an integrated circuit whereby an endpoint can be readily detected.
The above and other needs are met by a method for planarizing an integrated circuit on a substrate to a target surface of the substrate where at least portions of the target surface are of a first material having a first reflectivity. The substrate is overlaid with a top layer of a second material having a second reflectivity, thereby forming an upper surface. Material is removed from the upper surface in a planarizing process, and the first reflectivity and second reflectivity of the upper surface are sensed with multiple wavelengths of electromagnetic radiation. The planarization process is stopped when a ratio of the second reflectivity to the first reflectivity equals a predetermined value.
Thus, by sensing the first reflectivity, the second reflectivity, or a combination of the first and second reflectivity with multiple wavelengths of electromagnetic radiation, a planarization process endpoint can be readily detected. If only one wavelength of electromagnetic radiation is used, the change or reversal in reflectivity may not be sensed in some portions of the thickness cycle of the first material. By using multiple wavelengths of electromagnetic radiation, the combined cycle of the multiple wavelengths overlap and at least one wavelength will sense the change in reflectivity in all portions of the thickness cycle of the first material, thereby preventing too much or too little material from being removed from the substrate.
In various preferred embodiments of the invention, the first material is silicon oxide and the second material is titanium nitride. The planarization process for removing material from the substrate is preferably chemical mechanical polishing, with use of an oxide slurry. The first and second reflectivities are preferably sensed by at least three wavelengths of electromagnetic radiation, which originate from a single lasing device. In an alternate embodiment, the multiple wavelengths originate from at least three lasing devices. Most preferably, the sum of any two of the multiple wavelengths of electromagnetic radiation is greater than any one of the multiple wavelengths. The planarization process is preferably stopped when the predetermined value of the ratio of the second reflectivity to the first reflectivity equals about zero, which indicates that all of the second material has been removed.